George McGovern, a party elder who was one of Hillary Clinton's earliest and most prominent supporters, called on her yesterday to quit the Democratic race, raising the risk of further defections after her disappointing performance in North Carolina and Indiana.

McGovern, the former Democratic presidential candidate and a friend of both Clintons for more than 30 years, became the most significant figure yet to call for her to stand aside, saying it was virtually impossible for her to win the nomination against Barack Obama.

"I think that the mathematics indicate that Senator Obama is probably going to be the Democratic presidential nominee," McGovern told MSNBC. "The time has come from Democrats to unite to get ready for a tough race this fall against [Republican] Senator [John] McCain."

McGovern said he told Bill Clinton yesterday that he would support Obama. His decision threatened to set off a widespread defection of superdelegates to Obama that could end Clinton's race for the White House before the primary season wraps up on June 3.

Clinton insisted yesterday that she intended to fight on, in spite of the arithmetic being against her and severe funding problems. She revealed she had lent her campaign a further $6.4m (£3.3m) and, with virtually no chance of winning, she is unlikely to attract many donors. The money comes on top of $5m she lent in February. Her team said yesterday that she was prepared to make further loans.

Speaking in West Virginia, which holds its primary next week, Clinton defended her narrow win in Indiana and argued that recent victories in industrial states showed she would be a stronger candidate. "I'm staying in this race until there is a nominee," she said. "We should stay focused on nominating the stronger candidate against John McCain."

But despite her assertions, the dynamics of the contest changed after Tuesday's primary results, with Obama beginning to be treated by senior Democrats as the presumptive nominee. He beat Clinton 56% to 42% in North Carolina and she won Indiana, but only narrowly, 51% to 49%.

In a conference call with reporters, David Plouffe, Obama's campaign manager, said: "We can see the finish line."

Only six contests remain on the primary calendar, making it virtually impossible for Clinton to catch up in delegates or the popular vote. She has few good options. She is pressing the Democratic National Committee to count the results of unauthorised primary elections in Florida and Michigan, which could help her. However, that would be strenuously opposed by Obama. The DNC's rules committee is to discuss Florida and Michigan on May 31.

Tuesday's results gave Obama his biggest lead over Clinton since voting began. Overall, he now has the support of 1,840 of the 2,025 delegates needed to secure the nomination, according to the Associated Press. Clinton has 1,688. Obama took 66 of the delegates to her 49 in North Carolina and she took 38 to his 34 in Indiana, giving him an extra 13 overall on the night.

The focus now shifts to the remaining 260-plus undecided superdelegates - Congress members and others with an automatic vote. Obama's campaign urged them to come off the fence and is to meet uncommitted superdelegates today.

Clinton's campaign called a meeting of her own with the uncommitted superdelegates yesterday in an attempt to head off a mass movement towards Obama. But her efforts were undermined by the call from McGovern. It was also a personal rebuff for Clinton, who cut her teeth in politics working on McGovern's campaign in Texas with her then boyfriend, Bill. More than 30 years later McGovern returned the favour with a warm endorsement in Iowa last autumn. At the time, McGovern had said Clinton would make an even better president than her husband.

In another indication that a large-scale shift in superdelegates towards Obama may be under way, Rahm Emanuel, a former Clinton administration aide who is now a Chicago area congressman, issued a veiled warning on the importance of maintaining party loyalty.

"How the loser loses will determine whether the winner can go on and win," he told National Public Radio yesterday. "If you look at history whether it was Ted Kennedy in 1980 versus Jimmy Carter in that primary or Ronald Reagan in '76 with Gerald Ford, both of those candidates, how they lost their spirited primaries affected the winner's ability to move on and have an effective general election."

His warnings about the risks of a divided party were underlined by exit polls. A third of Clinton supporters say they would vote for McCain over Obama next November, suggesting the acrimonious exchanges between the Democratic contenders could hurt the party's chances.

Despite such warnings, there was no sign from Clinton that she would do anything other than fight on. After yesterday's appearances in West Virginia, she plans to campaign today in Oregon, which holds its primary on May 20, and South Dakota, which votes on June 3.